Network tap module

ABSTRACT

A network tap module having a network line monitoring function has a signal splitter to split a signal received from a network into a first copy which is returned to the network and a second copy. A line monitoring and statistics circuit receives the second copy of the signal and carries out line monitoring and statistics collecting thereon. The module further has at least one of: (i) a display for displaying an indication of the state of the network line based on the line monitoring and statistics collection carried out by the line monitoring and statistics circuit, and (ii) an interface for allowing data relating to the state of the network line based on the line monitoring and statistics collection carried out by the line monitoring and statistics circuit to be output from the module.

A tap is typically deployed on a link between network elements in orderto provide an access point where instrumentation, typically a “protocolanalyser” device, may be attached to the network serial link withoutdisruption at the protocol, electrical or optical levels. This isgenerally performed in order to capture and sometimes visualise networktraffic for diagnostic or characterisation purposes.

Two types of serial line tap have been implemented to date, namelyoptical taps and electrical taps.

Optical taps operate entirely in the optical domain. Two principalmethods have been implemented. In the first, known as medium splitting,a portion of the physical medium (glass fibre) is “peeled off” toprovide a line tap. In the second, known as optical splitting, a portionof the light is redirected by a reflection or refraction mechanism toprovide a line tap. These optical taps are generally implemented as adiscrete module that provides a pair of standard optical connectors,such as LC, to connect “in-line” into the serial line to be tapped. Bothof these methods of optical tapping result in attenuation of the ongoingsignal level on the tapped line.

Electrical taps employ some type of signal splitter circuit which isinserted into a serial electrical transmission line providing anexternal electrical access point. Such taps can be implemented ininstrumentation devices or in a pluggable transceiver module, such as aGigabit Interface Converter (GBIC). Electrical taps may or may notinfluence the electrical signal integrity of the ongoing serial line.

Traditionally, problems on a network link or line have been located anddiagnosed as follows. Once it has been appreciated that a network is notoperating correctly, a tap is connected to a link of the network and anetwork analyser is connected to the tap. In this way, signals tapped bythe tap are passed to the analyser for analysis. If it is found thatthat particular link is operating correctly, the analyser and tap areremoved and connected to the network on a different link. Alternativelymultiple taps may be deployed at strategic positions in the network andthe analyser moved from one tapped link to the next. Because theoperator of the network will often have little or no idea of exactlywhere the errors are occurring, it may be necessary to check severallinks before the link having the errors is located. It will beappreciated immediately that this can be a very labour-intensive andtime-consuming exercise, particularly considering that a large networkmay consist of hundreds or thousands of connected devices and thereforehundreds or thousands of links. Moreover, the mere act of connecting atap or analyser in this way may be sufficient to break the network link,which can cause the link to be initialised, which can inadvertentlyremove the error. If that happens, then the analyser will miss the factthat it was that link that had errors.

According to a first aspect of the present invention, there is provideda network tap module having a network line monitoring function, themodule comprising: a signal splitter constructed and arranged to split asignal, which is received from a network to which in use the tap moduleis connected, into a first copy which is returned to the network and asecond copy; a line monitoring and statistics circuit constructed andarranged to receive the second copy of the signal from the signalsplitter and to carry out line monitoring and statistics collectingthereon; and, at least one of: (i) a display for displaying anindication of the state of the network line based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit, and (ii) an interface for allowing data relating tothe state of the network line based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit to be output from the module.

This aspect of the present invention is particularly useful for tappingan electrical network. In another aspect discussed further below, thereis provided an optical network tap module. In any case, in accordancewith the present invention, the tap modules have a monitoring andstatistics capability so that errors can be indicated on a local displayof the module and/or enable a remote alert to be raised and output viathe interface, which permits remote reporting and control. Typically,the errors will include signal, transmission word and frame levelerrors. Preferably, the tap module performs serial line analysis. Thetap module, which can be small enough effectively to be hand-held, canbe relatively inexpensive. This means that a large number of the tapmodules can be deployed, permanently or semi-permanently, around anetwork so that the logical state of the network can be monitored atseveral or many points more or less continuously. If an error isindicated by a particular tap module, the operator can simply physicallygo to the tap module that is indicating or reporting an error and, inthe preferred embodiment, can plug a network analyser directly into thetap module in order to carry out detailed analysis. It is not necessaryfor the link to be broken by the act of plugging in or removing theanalyser in this preferred embodiment, thus avoiding the problem ofinitialisation of the link mentioned above. A network tap module inaccordance with the present invention can be regarded as a “tap withintelligence”, compared to the rather “dumb” taps of the prior art, inthe sense that the network tap module of the present invention itselfprovides an indication or report of errors. In one preferred embodiment,where many network tap modules are deployed around a network, it isconvenient for those network tap modules to be connected to a reportingstation at which a operator can be provided with a report of any errorsindicated by any of the tap modules so that the operator can quickly beprovided with information about which network tap module is reportingerrors on the network being monitored.

In a preferred embodiment, the network tap module comprises both adisplay for displaying an indication of the state of the network linebased on the line monitoring and statistics collection carried out bythe line monitoring and statistics circuit, and an interface forallowing data relating to the state of the network line based on theline monitoring and statistics collection carried out by the linemonitoring and statistics circuit to be output from the module.

The network tap module preferably comprises a retimer circuitconstructed and arranged to receive and regenerate the first copy ofsaid signal prior to that copy being passed back into a said network.This restores signal integrity, especially signal amplitude and timing,such that the ongoing signal passed back to the network is not degraded.This is particularly useful in electrical networks.

The network tap module may comprise an output line on which a third copyof said signal is in use output. Decoupling and/or protectionappropriate to the serial technology being tapped may be implemented inthis line.

According to a second aspect of the present invention, there is provideda network tap module having a network line monitoring function, themodule comprising: an optical signal splitter constructed and arrangedto split an optical signal, which is received from an optical network towhich in use the tap module is connected, such that a first copy of thesignal continues along the optical network without retiming and toprovide a second copy of the optical signal; a line monitoring andstatistics circuit constructed and arranged to receive the second copyof the signal from the signal splitter and to carry out line monitoringand statistics collecting thereon; and, at least one of: (i) a displayfor displaying an indication of the state of the network line based onthe line monitoring and statistics collection carried out by the linemonitoring and statistics circuit, and (ii) an interface for allowingdata relating to the state of the network line based on the linemonitoring and statistics collection carried out by the line monitoringand statistics circuit to be output from the module.

In this aspect, the optical splitter takes a proportion, say 10%, of thelight from the optical network, which is fed to the line monitorcircuit. The remainder of the light, in this example 90%, continues downthe tapped serial line.

The network tap module preferably comprises both a display fordisplaying an indication of the state of the network line based on theline monitoring and statistics collection carried out by the linemonitoring and statistics circuit, and an interface for allowing datarelating to the state of the network line based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit to be output from the module.

The network tap module may comprise an optical receiver constructed andarranged to receive the second copy of the signal from the signalsplitter and to convert the received copy from optical to electricalformat prior to passing it to the line monitoring and statisticscircuit.

The network tap module may comprise an output line on which a third copyof said signal is in use output. Decoupling and/or protectionappropriate to the serial technology being tapped may be implemented inthis line.

According to a third aspect of the present invention, there is provideda network tap module, the network tap module comprising: a firstconnector for connecting the module to a first network serial line sothat a signal can be received at the first connector from a said firstnetwork serial line; a second connector for connecting the module to asecond network serial line so that a signal can be received at thesecond connector from a said second network serial line; a first signalsplitter constructed and arranged to receive a signal from a said firstnetwork serial line via the first connector and to produce at least twosubstantially identical copies of said signal; a second signal splitterconstructed and arranged to receive a signal from a said second networkserial line via the second connector and to produce at least twosubstantially identical copies of said signal; a first retimer circuitconstructed and arranged to receive a first of said copies of saidsignal from the first signal splitter and to regenerate said signal forpassing back into a said first network serial line; a second retimercircuit constructed and arranged to receive a first of said copies ofsaid signal from the second signal splitter and to regenerate saidsignal for passing back into a said second network serial line; a linemonitoring and statistics circuit constructed and arranged to receive asecond of said copies of said signal from the first signal splitter andto carry out line monitoring and statistics collecting thereon, and toreceive a second of said copies of said signal from the second signalsplitter and to carry out line monitoring and statistics collectingthereon; and, at least one of: (i) a display for displaying anindication of the state of said network serial lines based on the linemonitoring and statistics collection carried out by the line monitoringand statistics circuit, and (ii) an interface for allowing data relatingto the state of said network serial lines based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit to be output from the module.

In this aspect, a pair of active monitoring serial line taps isimplemented, preferably in a single physical module, so that tap portsand serial line events and statistics can be provided for each serialline comprising a duplex serial line pair. The network tap module has amonitoring and statistics capability so that errors can be indicated ona local display and/or that enables a remote alert to be raised andoutput via the interface, which permits remote reporting and control.Typically, the errors will include signal, transmission word and framelevel errors. The retiming functions restore signal integrity such thatthe ongoing signals passed back to the network are not degraded. This isthe case regardless of whether the network tap module is acting as anelectrical tap or an optical tap. Thus, even when tapping an opticalnetwork, in this aspect there is no attenuation of the ongoing signal.Again, plural such network tap modules can be permanently orsemi-permanently deployed around a network being monitored. An operatorcan simply plug a network analyser into any one of the network tapmodules if that network tap module is for example indicating orreporting an error. Again, this means that a relatively inexpensive itemcan be left permanently or semi-permanently in place in a network andthat item itself reports or indicates the presence of errors in thenetwork.

The network tap module preferably comprises both a display fordisplaying an indication of the state of said network serial lines basedon the line monitoring and statistics collection carried out by the linemonitoring and statistics circuit, and an interface for allowing datarelating to the state of said network serial lines based on the linemonitoring and statistics collection carried out by the line monitoringand statistics circuit to be output from the module.

Preferably, the first retimer circuit is constructed and arranged topass said regenerated signal back into a said first network serial linevia the second connector. The first connector is preferably constructedand arranged to convert a said signal from a said first network serialline from a first format to a second format, and the second connector isconstructed and arranged to convert said regenerated signal into saidfirst format prior to said regenerated signal being passed back into asaid first network serial line. In an example, the first connectorprovides a standard electrical signal as an output regardless of whetherthe serial line being tapped is optical or electrical, the regeneratedsignal being converted back as necessary by the second connector.

Preferably, the second retimer circuit is constructed and arranged topass said regenerated signal back into a said second network serial linevia the first connector. The second connector is preferably constructedand arranged to convert a said signal from a said second network serialline from a first format to a second format, and the first connector isconstructed and arranged to convert said regenerated signal into saidfirst format prior to said regenerated signal being passed back into asaid second network serial line. In an example, the second connectorprovides a standard electrical signal as an output regardless of whetherthe serial line being tapped is optical or electrical, the regeneratedsignal being converted back as necessary by the first connector.

The tap preferably comprises respective output lines on which copies ofsaid signal produced by the first and second signal splitters can berespectively output. Decoupling and/or protection appropriate to theserial technology being tapped may be implemented in these lines.

Preferably, the or each connector is a pluggable transceiver module.This allows the network tap module to be deployed in multi-mode optical,single-mode optical and electrical serial transmission lines.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 shows schematically an example of a network tap module forin-line electrical operation in accordance with an embodiment of thepresent invention;

FIG. 2 shows schematically an example of a network tap module foroptical operation in accordance with an embodiment of the presentinvention;

FIG. 3 shows schematically an example of a preferred embodiment of afull duplex active monitoring serial line tap module in accordance withthe present invention; and,

FIG. 4 shows schematically an example of a preferred embodiment of aline monitoring and statistics circuit for use with the tap modules ofFIGS. 1 to 3.

FIG. 1 shows schematically an example of an active monitoring serialline tap 100 in accordance with an embodiment of the present invention.SL 101 represents the serial line to be tapped. SL 101 is fed to asignal splitting integrated circuit 102 of the tap 100, which provides aminimum of three serial outputs 103,104,105, all of which aresubstantially identical to the SL input. The signal splitting circuit102 may be implemented for example using a crosspoint switch, a portbypass circuit package (PBC), also referred to as a link resiliencycircuit (LRC), or by a custom design. Output 103 is a serial line tapwhich exits the enclosure 112 of the tap 100 via an electrical connector113. Decoupling and/or protection appropriate to the serial technologybeing tapped may be implemented in this line. Output 105 represents theongoing serial line which is fed to a retimer circuit 109 whichregenerates the signal, restoring signal amplitude and timing as isknown in active taps per se. Output 111 (SL_(RT)) from the retimer 109represents the ongoing serial line which exits the enclosure 112. Output104 from the signal splitter 102 is fed into a line monitor andstatistics circuit 106. This monitoring and statistics logic 106 may beimplemented using an FPGA or ASIC, for example. Monitored events andstatistics provided by the line monitor and statistics logic 106 mayinclude, but are not restricted to, the list shown in Table 1. TABLE 1Line rate Loss of signal Loss of synchronisation Invalid transmissionwords (invalid code or disparity error) Transmission word error rate(word errors per second) Link Initialisation primitive signals andsequences Percentage bandwidth utilisation Frame rate (frames persecond) Maximum frame size Data rate (MB/s) CRC and Checksum errorsFrame errors (invalid Start of Frame/invalid length) Transport Protocoltypes Classes of service or QoS (Quality of Service) Priority

The line monitor and statistics circuit 106 drives an integral display107 of the tap 100, which may be implemented using LEDs, LCD or anyother suitable display type. This display 107 gives a clear indicationof the state of the tapped serial line SL101, showing events andstatistics collected and computed by the line monitor and statisticscircuit 106. These events and statistics collected and computed by theline monitor and statistics circuit 106 are also fed to a networkinterface circuit 108, which may be for example an Ethernet controlleror other network technology appropriate to the network environment. Anetwork output 110 from the network interface circuit 108 provides aconnection to a network (not shown), thus permitting remote reporting ofevents and statistics collected and computed by the line monitor andstatistics circuit 106 and also permitting remote access forconfiguration and/or control purposes to the active monitoring serialline tap 100.

FIG. 2 shows schematically an example of a monitoring serial line tap200 in accordance with an embodiment of the present invention, deployedpassively in an optical environment using an optical splitter. OpticalSL 215 represents the optical serial line to be tapped. The opticalsplitter 214, which in this case is a passive device, “splits” a portionof light from optical SL 215, which is fed to an optical receiver 213 ofthe tap 200. The optical receiver 213 receives the optical bit streamand converts it to an electrical bit stream which is fed via a serialline 201 to a signal splitting integrated circuit 202. The signalsplitting integrated circuit 202 may be implemented for example using acrosspoint switch, a port bypass circuit package(PBC), also referred toas a link resiliency circuit (LRC), or by a custom design. Output 203 isa serial line tap which exits the enclosure 212 of the tap 200 viaelectrical connector 216. Decoupling and/or protection appropriate tothe serial technology being tapped may be implemented in this line.Output 204 from the signal splitter 202 is fed into a line monitor andstatistics circuit 206. This monitoring and statistics circuit 206 maybe implemented using an FPGA or ASIC, for example. Monitored events andstatistics provided by the line monitor and statistics circuit 206 mayinclude, but are not restricted to, the list shown in Table 1.

The line monitor and statistics circuit 206 drives an integral display207, which may be implemented using LEDs, LCD or any other suitabledisplay type. This display 207 gives a clear indication of the state ofthe tapped serial line 215, showing events and statistics collected andcomputed by the line monitor and statistics circuit 206. These eventsand statistics collected and computed by the line monitor and statisticscircuit 206 are also fed to a network interface circuit 208, which maybe for example an Ethernet controller or other network technologyappropriate to the network environment. A network output 210 from thenetwork interface circuit 208 provides a connection to a network (notshown), thus permitting remote reporting of events and statisticscollected and computed by the line monitor and statistics circuit 206and also permitting remote access for configuration and/or controlpurposes to the monitoring serial line tap 200.

FIG. 3 shows schematically an example of a preferred embodiment of amonitoring serial line tap 300 in accordance with the present invention.In this embodiment, a pair of active monitoring serial line taps isimplemented in a single physical module so that tap ports and serialline events and statistics can be provided for each serial linecomprising a duplex serial line pair SL1,SL2.

The duplex tap 300 is connected into the duplex serial line pair SL1,SL2that is to be tapped via small form factor pluggable transceiver modulesSFP 1 305, SFP 2 306. These SFPs 305,306 may be multi-mode optical,single-mode optical or electrical, depending on the medium type of theduplex line to be tapped. The SFPs 305,306 can be swapped from one typeto another depending on the type of network to be monitored.

Consider first one half SL1 of the duplex serial line pair SL1,SL2. SL1301 plugs into SFP 1 305. The output SL1 307 of SFP 1 305 provides astandard electrical signal regardless of whether the serial line SL1 301being tapped is optical or electrical. The output SL1 307 is fed into afirst signal splitter circuit 311. Signal splitter circuit 311 providesthree outputs identical to its SL1 input 307. Output 315 is the serialline tap for SL1 that is made externally accessible via electricalconnector 326. Output 317 is fed to a first retimer 312 whichregenerates SL1 as SL1 _(RT) 309, restoring signal amplitude and timingas is known in active taps per se. In a preferred embodiment, the signalsplitter 311 and retimer functions 312 are combined into a singleintegrated circuit. Retimed signal SL1 _(RT) 309 is fed to thetransmitter side of SFP 2 306, which performs any necessary conversionto the medium type of the tapped serial line 301/303.

Correspondingly, the other half SL2 304 of the duplex serial line pairplugs into SFP 2 306 and is fed into a second signal splitter circuit314. Output 320 is the serial line tap for SL2 that is made externallyaccessible via electrical connector 326. Output 318 is fed to a secondretimer 313, and the retimed output SL2 _(RT) 308 is fed to thetransmitter side of SFP 1 305, which performs any necessary conversionto the medium type of the tapped serial line 302/304.

Outputs 316,319 from the first and second signal splitters respectivelyare both fed into a line monitoring and statistics circuit 321. In anembodiment that is intended for use on Fibre Channel links running at1.0625 and 2.125 GHz, the events and computed statistics shown in Table1 are collected and computed for both SL1 and SL2.

The line monitoring and statistics circuit 321 drives an integraldisplay 322, which again may be implemented using LEDs, LCD or any othersuitable display type. In one embodiment of the present invention, thevalues displayed are those shown in Table 2 (which also applies equallyto the other examples described above). TABLE 2 Line condition Line rate(1 or 2 Gb/s) Loss of signal Loss of synchronisation GBIC type (1G/2G,SW/LW/Cu) - this implies link type Invalid transmission words (invalidcode or disparity error) Transmission word error rate (word errors/s)Primitive Loop Initialisation primitives (all LIP types) traffic Pointto point initialisation primitive (LR and LRR) Point to point offlineand not operational primitives (NOS and OLS) Frame Traffic Percentagebandwidth utilisation Frame rate (frames per second) Maximum frame size(512 B/1 kB/2 kB) Data rate (MB/s) CRC errors Frame errors (invalidSOF/length/data word) Protocol types (SCSI/IP/FICON/Link Service/Other)Classes of service (1/2/3/4)

The line monitoring and statistics circuit 321 also formats and sendsevents and computed statistics to a network interface 323, such as a10/100 Mbit Ethernet MAC/PHY 323. The format of these statistics may beSNMP/RMON and/or proprietary. The network attachment is made by a link324 which is externally accessible via an electrical connector.

FIG. 4 shows schematically an example of a preferred embodiment of aline monitoring and statistics circuit 400, which may be used as theline monitoring and statistics circuit 106,206,321 in the tap modules100,200,300 described above. The embodiment shown in FIG. 4 representsschematically the functions required for performing monitoring andstatistics collection for a single serial line, SL. The preferredembodiment of the present invention implements exactly two suchcircuits, one for each of the serial lines representing a duplex pair.In the embodiment shown, the line monitoring and statistics function isimplemented using a field programmable gate array (FPGA).

Serial Line SL 401 is input to deserialiser 402. The function of thedeserialiser 402 is to convert the serial bit stream input to a 10 bitwide parallel character stream 403. The deserialiser 402 is capable ofdetecting loss of signal and loss of synchronisation, which are bothindications of potential network problems. In the event of an occurrenceof a loss of synchronisation or loss of signal, the correspondingcounter within a counter and statistic store 415 is incremented.

The 10 bit wide parallel character stream 403 is input to a 10 bit-to-8bit decoder 404. The 10 bit-to-8 bit decoder 404 converts the 10 bitparallel character stream 403 into an 8 bit parallel character stream405. The 10 bit-to-8 bit decoder 404 is capable of detecting disparityerrors and code violations, which are both indications of potentialnetwork problems. In the event of an occurrence of a disparity error ora code violation, the corresponding counter within the counter andstatistic store 415 is incremented.

The 8 bit wide parallel character stream 405 is input to an ordered setdetector 406. The ordered set detector 406 assembles groups of four 8bit characters into 32 bit transmission words and compares the 32 bitpatterns with a number of preconfigured 32 bit patterns which aredefined in the Fibre Channel ANSI standards as ordered sets. Thisprocess enables the network topology type to be determined. The networktopology type is stored in the counter and statistic store 415.Initialisation events may also be detected by this process. In the eventof an occurrence of an initialisation of the monitored serial line 401being detected, the corresponding counter within the counter andstatistic store 415 is incremented. Transmission words are output as a32 bit wide transmission word stream 407.

The transmission word stream 407 is input to a frame detector,classifier, CRC checker 408 which translates the transmission wordstream into frames. A number of operations are carried out hereincluding, but not restricted to: checking of frame delimiters,recalculation and checking of CRC, counting of frames and characterswithin frames, frames size determination, class of servicedetermination, protocol identification. Corresponding counters withinthe counter and statistic store 415 are incremented accordingly.

A collector and formatter 413 periodically reads the contents of thecounter and statistic store 415 and formats the information for outputto the network interface 417 and the local display interface 416.

The preferred embodiments described herein provide a tap which may beused in an optical or electrical serial transmission line (or pluralityof serial transmission lines) and which incorporates a line monitoringfunction in conjunction with a local display and/or an external networklink. The line monitoring function analyses the tapped serial line atthe signal, transmission word and frame levels, and provides anindication of a plurality of error and computed statistical informationat the local display or via the optional network link so that anoperator can attach a more sophisticated network analyser (to performdetailed analysis) to the tap port provided. The operator does not haveto break a link merely in order to be able to attach the analyser and sothe detected errors should still be present on the link when theanalyser is connected as the link is not initialised. Plural such tapscan be deployed around the network, as discussed above. The specificserial line tap disclosed here is intended for use in any serial networkenvironment. A serial network environment has network nodes (orend-points) that are connected using electrical cable and/or fibre opticcable and may include a plurality of switches, bridges, hubs, routersand gateways.

This tap is applicable to, but not restricted to, the following networkenvironments: local area networks (LAN), storage area networks (SAN),system area networks (SAN), metropolitan area networks (MAN), and widearea networks (WAN).

This tap is applicable to, but not restricted to, the following networkphysical transport technologies:

-   Ethernet (10/100/1,000/10,000 Mb/s), OC3/12/48/192/768,-   Fibre Channel (1.0625/2.125/4.25/12.75 MB/s), and-   InfiniBand (2.5 Gb/s, 1×/4×/12×).

Embodiments of the present invention have been described with particularreference to the examples illustrated. However, it will be appreciatedthat variations and modifications may be made to the examples describedwithin the scope of the present invention.

1. A network tap module having a network line monitoring function, themodule comprising: a signal splitter constructed and arranged to split asignal, which is received from a network to which in use the tap moduleis connected, into a first copy which is returned to the network and asecond copy; a line monitoring and statistics circuit constructed andarranged to receive the second copy of the signal from the signalsplitter and to carry out line monitoring and statistics collectingthereon; and, at least one of: (i) a display for displaying anindication of the state of the network line based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit, and (ii) an interface for allowing data relating tothe state of the network line based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit to be output from the module.
 2. A network tap module accordingto claim 1, comprising both a display for displaying an indication ofthe state of the network line based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit, and an interface for allowing data relating to the state of thenetwork line based on the line monitoring and statistics collectioncarried out by the line monitoring and statistics circuit to be outputfrom the module.
 3. A network tap module according to claim 1 [or claim2], comprising a retimer circuit constructed and arranged to receive andregenerate the first copy of said signal prior to that copy being passedback into a said network.
 4. A network tap module according to claim 1[any of claims 1 to 3], comprising an output line on which a third copyof said signal is in use output.
 5. A network tap module having anetwork line monitoring function, the module comprising: an opticalsignal splitter constructed and arranged to split an optical signal,which is received from an optical network to which in use the tap moduleis connected, such that a first copy of the signal continues along theoptical network without retiming and to provide a second copy of theoptical signal; a line monitoring and statistics circuit constructed andarranged to receive the second copy of the signal from the signalsplitter and to carry out line monitoring and statistics collectingthereon; and, at least one of: (i) a display for displaying anindication of the state of the network line based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit, and (ii) an interface for allowing data relating tothe state of the network line based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit to be output from the module.
 6. A network tap module accordingto claim 5, comprising both a display for displaying an indication ofthe state of the network line based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit, and an interface for allowing data relating to the state of thenetwork line based on the line monitoring and statistics collectioncarried out by the line monitoring and statistics circuit to be outputfrom the module.
 7. A network tap module according to claim 5 [or claim6], comprising an optical receiver constructed and arranged to receivethe second copy of the signal from the signal splitter and to convertthe received copy from optical to electrical format prior to passing itto the line monitoring and statistics circuit.
 8. A network tap moduleaccording to claim 5 [any of claims 5 to 7], comprising an output lineon which a third copy of said signal is in use output.
 9. A network tapmodule, the network tap module comprising: a first connector forconnecting the module to a first network serial line so that a signalcan be received at the first connector from a said first network serialline; a second connector for connecting the module to a second networkserial line so that a signal can be received at the second connectorfrom a said second network serial line; a first signal splitterconstructed and arranged to receive a signal from a said first networkserial line via the first connector and to produce at least twosubstantially identical copies of said signal; a second signal splitterconstructed and arranged to receive a signal from a said second networkserial line via the second connector and to produce at least twosubstantially identical copies of said signal; a first retimer circuitconstructed and arranged to receive a first of said copies of saidsignal from the first signal splitter and to regenerate said signal forpassing back into a said first network serial line; a second retimercircuit constructed and arranged to receive a first of said copies ofsaid signal from the second signal splitter and to regenerate saidsignal for passing back into a said second network serial line; a linemonitoring and statistics circuit constructed and arranged to receive asecond of said copies of said signal from the first signal splitter andto carry out line monitoring and statistics collecting thereon, and toreceive a second of said copies of said signal from the second signalsplitter and to carry out line monitoring and statistics collectingthereon; and, at least one of: (i) a display for displaying anindication of the state of said network serial lines based on the linemonitoring and statistics collection carried out by the line monitoringand statistics circuit, and (ii) an interface for allowing data relatingto the state of said network serial lines based on the line monitoringand statistics collection carried out by the line monitoring andstatistics circuit to be output from the module.
 10. A network tapmodule according to claim 9, comprising both a display for displaying anindication of the state of said network serial lines based on the linemonitoring and statistics collection carried out by the line monitoringand statistics circuit, and an interface for allowing data relating tothe state of said network serial lines based on the line monitoring andstatistics collection carried out by the line monitoring and statisticscircuit to be output from the module.
 11. A network tap module accordingto claim 9 [or claim 10], wherein the first retimer circuit isconstructed and arranged to pass said regenerated signal back into asaid first network serial line via the second connector.
 12. A networktap module according to claim 11, wherein the first connector isconstructed and arranged to convert a said signal from a said firstnetwork serial line from a first format to a second format, and thesecond connector is constructed and arranged to convert said regeneratedsignal into said first format prior to said regenerated signal beingpassed back into a said first network serial line.
 13. A network tapmodule according to claim 9 [any of claims 9 to 12], wherein the secondretimer circuit is constructed and arranged to pass said regeneratedsignal back into a said second network serial line via the firstconnector.
 14. A network tap module according to claim 13, wherein thesecond connector is constructed and arranged to convert a said signalfrom a said second network serial line from a first format to a secondformat, and the first connector is constructed and arranged to convertsaid regenerated signal into said first format prior to said regeneratedsignal being passed back into a said second network serial line.
 15. Anetwork tap module according to claim 9 [any of claims 9 to 14],comprising respective output lines on which copies of said signalsproduced by the first and second signal splitters can be respectivelyoutput.
 16. A network tap module according to claim 9 [any of claims 9to 15], wherein the or each connector is a pluggable transceiver module.17. A network tap module according to claim 9, wherein the first retimercircuit is constructed and arranged to pass said regenerated signal backinto a said first network serial line via the second connector; thefirst connector is constructed and arranged to convert a said signalfrom a said first network serial line from a first format to a secondformat, and the second connector is constructed and arranged to convertsaid regenerated signal into said first format prior to said regeneratedsignal being passed back into a said first network serial line; thesecond retimer circuit is constructed and arranged to pass saidregenerated signal back into a said second network serial line via thefirst connector; and, the second connector is constructed and arrangedto convert a said signal from a said second network serial line from afirst format to a second format, and the first connector is constructedand arranged to convert said regenerated signal into said first formatprior to said regenerated signal being passed back into a said secondnetwork serial line.
 18. A network tap module according to claim 17,comprising respective output lines on which copies of said signalsproduced by the first and second signal splitters can be respectivelyoutput.
 19. A network tap module according to claim 17, wherein the oreach connector is a pluggable transceiver module.